Generally, conventional machine tools such as bending machines for bending a plate-shaped workpiece (material) are configured to the bending by: detecting a pinch position (the position of the upper surface of the workpiece) at which the workpiece is clamped with a die and a punch; calculating a punch final depth as a position at which the bending starting from the detected pinch position finishes, based on the pinch position; and pushing in the punch to the punch final depth.
Note that the depth of the punch and the bend angle of the workpiece are closely related. To accurately bend the workpiece at a given angle, it is important to obtain an accurate punch final depth.
As this type of technique, those described in literatures listed below have heretofore been known, for example (Japanese Patent Application Publication Nos. Hei 6-134524, 2000-140943, and Hei 8-24955).